1. Field of the Invention
The present invention relates to a blood pump used for example in a ventricular assist device, and particularly relates to a blood pump having a construction that suppresses the development of thrombi.
2. Related Art
Axial flow, diagonal flow and centrifugal pumps are known as blood pumps for ventricular assist devices. As such a blood pump needs to be of a compact design in order to fit into the human body, its surface area needs to be large compared to its internal cubic capacity, and the clearance between the impellers and the casing needs to be large compared to the diameter. Therefore, substantial energy losses are incurred and the pump efficiency is considerably lower than that of ordinary larger pumps for industrial use.
Among the blood pumps stated above, centrifugal pumps are characterized by that the aspirated blood will hit perpendicularly upon the pump base part and will then be re-directed by centrifugal force in a right angle in the direction of the circumference. In this way, centrifugal pumps efficiently turn fluid motion energy into pressure energy by means of centrifugal force. Further, a hemolysis decrease can be expected due to the lower circumferential/angular velocity of centrifugal pumps as compared to axial pumps.
According to whether they feature a back plate (shroud), centrifugal pumps can be generally classified into open vane type, semi-open vane type and closed type centrifugal pumps.
Blood pumps are naturally restricted by the properties of blood. If blood stagnates, comes into contact with air or foreign bodies over a longer period of time, or is subjected to a higher temperature environment, it coagulates and thrombi form. Furthermore, the thrombi grow on these grounds. Blood properties like these form the reason for limitations regarding the blood pump construction.
The above-mentioned centrifugal pumps of the closed type or semi-open type feature a construction wherein the impellers are fitted to the back plate, and between the back plate and the pump base part the pump function does not take effect. Therefore, if closed type or semi-open type centrifugal pumps were used as a blood pump, the blood would stagnate at the backside of the back plate, leading to the development of thrombi. They are thus not appropriate as blood pumps.
In contrast, open vane type centrifugal pumps as shown in FIG. 7A feature no back plate, the impellers being directly fitted to the rotation shaft tip, and therefore allow free flow of the blood in areas where thrombi have a tendency to develop in blood pump types featuring back plates.
Conventional blood pumps however, in order to yield a high pump efficiency, feature a construction wherein the impellers spread from the periphery of the boss in a uniform manner, and the blood flowing in the “vicinity of the periphery of the boss parts (boss, rotating side of the mechanical seal)” as exemplified by the inside of the chain double-dashed line of FIG. 7B is of a lower velocity than the blood in the radial outer areas and is of about the same velocity as the boss part.
Therefore, thrombi are likely to form inside the circle marked by the chain double-dashed line, as the blood is remaining in contact with parts that are foreign bodies to the blood (such as the boss and the impellers) over a longer period of time. When thrombi develop between the rotating part and the stationary part, the impeller rotation of the blood pump is obstructed. Furthermore, when the thrombi are scattered throughout the body, they end up clogging blood vessels. For example, when a thrombus reaches the brain it causes cerebral thrombosis.
Further, conventional blood pumps alter the blood current sharply, which is not necessarily desirable from a fluid dynamics point of view. According to conventional blood pumps, for example, the pump base part has a basically level surface on the pump part side, thus the blood is caused to stagnate and thrombi are likely to form.
In this way, the most important issue in blood pumps, as opposed to other fluid pumps, consists in solving the thrombus problem by all means.
It is the object of the present invention, in view of the above-mentioned circumstances, to provide a blood pump featuring a construction able to effectively prevent the development of thrombi.